Breath alcohol-measuring instruments have been increasingly used by police for checks in road traffic. To ensure the accuracy of measurement, especially with respect to subsequent legal proceedings, the breath alcohol-measuring instruments are calibrated at regular intervals. For calibration, it is necessary to provide a gas with a known ethanol concentration. Calibrating gas generators as they are described in, e.g., DE 32 16 109 C2 have been known for providing the test gas.
Prior-art calibrating gas generators have liquid containers which are filled with aqueous ethanol solutions of a known concentration. There also is provided air feed means, which draw in air by means of a pump and allows the air to escape in the liquid container, so that it bubbles through the ethanol solution. A line leads to the breath alcohol-measuring instrument from the gas atmosphere above the ethanol solution. While the air stream is bubbling through the solution in the form of fine bubbles, the air becomes enriched with ethanol and it becomes saturated with water vapor at the same time. The ethanol content in the air drawn off depends, according to Henry's law, on the concentration of ethanol in the ethanol solution and the temperature at which the ethanol is released from the liquid into the air.
The temperature dependence of the distribution coefficient K.sub.a/W, which indicates the ratio of the alcohol concentration in the air relative to that in the water, can be described by the following temperature dependence: EQU K.sub.a/W =0.04145.cndot.e.sup.a.T
in which T is the temperature in .degree.C. and a=0.0658 (1/.degree.C.). This temperature dependence is described in, e.g., the article AtemalkoholmeBgerate: Grundlagen der Kalibrierung [Breath Alcohol-measuring Instruments: Fundamentals of Calibration] by Gunter Schoknecht and Bruno Barduhn, Blutalkohol, Vol. 32/1995, pp. 1-9.
It follows from the dependence described above that the temperature of the solution must be maintained at a constant value with the smallest possible deviation in order to obtain the most stable ethanol concentration possible in the gas phase. If, e.g., a relative stability of 1% of the ethanol concentration in the gas phase is sought, the temperature must be maintained at a constant value within a range of .DELTA.T=0.01/a=0.15.degree. C.
To bring the temperature to a preset reference value, the prior-art calibrating gas generators were provided with thermostat and heating means in order to bring the aqueous ethanol solution to a reference temperature of 34.degree. C. and to carry out the calibration at this reference temperature. It was thus possible to perform the calibration at a preset, fixed reference temperature and known ethanol concentration in the aqueous solution.
The drawback of the prior-art procedure is that a mobile use of the calibration process is possible to a very limited extent only, because the heating means for the calibrating gas generators require an energy supply. Another drawback is a relatively long heat-up time required to bring the system into a stable, heated-up state. Condensation problems may also arise in the feed lines. On the whole, such a calibration process is technically rather complicated, because precise controls and heating means are necessary, which make the design of the instruments technically more complicated and, as a result, also more expensive.